In such apparatus, back lighting of the subject to be imaged usually has taken one of two forms or systems. In the first and more popular system, glass condenser lenses are employed in conjunction with a light source to collimate the light rays which are collected and focused by the objective onto the easel which holds the sensitized material or screen. In the alternative system, a light diffuser comprising either a sheet of opal glass or white translucent plastic is positioned between the light source and the material to be imaged.
Projection through condensers has been preferred because of the greater concentration of light with corresponding increased contrast and sharpness of image. In diffuse light projectors particularly those using pulsed xenon lamps, the high intensity of heat which accompanies the required light intensity has been a problem. Also, the condenser lenses when used with a point source light are incapable of covering as large an image area. In image projection for industrial purposes the availability of a large image in the negative becomes critical. For example, in the billboard industry where 150 line half tone film color separations may reach the size of 17 by 35 inches or even 26 by 35 inches, both of these sizes have to be blown up by a factor of 2.0 diameters to 5.0 diameters or even more. This must be done without sacrifice of dot quality. Obviously films as large as these cannot be enlarged with condenser projection because condensers that large are not presently available in the market place. There is also the problem of cost which places a practical limit on how large condensers can be made.
Photographic box lights which house pulsed xenon sources lamps have been used to backlight transparencies held in the subject holder of conventional two room "engraving" cameras. However, they have not lent themselves to use as a true projection source light in a single room projection operation where the source light, the film being projected, the objective lens, the necessary trackages and the easel which holds the sensitized material comprise an entity within four unpartitioned walls. This has been due to the problems of stray light leakage, overheating and operator eye safety.
Pulsed xenon source lamps of appropriate light intensity generate a tremendous amount of heat. This heat must be quickly and consistently dispelled with a fan or blower system which is either built into the cabinet which houses the lamps or is connected therewith by flexible hoses or other duct work. The air forced into the cabinet must also be allowed to escape. For this purpose, there must be adequate venting. Simple holes or louvers which comprise escape routes for the air make for an excessive amount of light leakage. This escaping light can be sufficient to create a light level in a dark room which cannot be tolerated if light sensitive materials are not to be light-struck.
The heat generated in a boxlight by such lamps is so intense that much of the box cannot be safely touched after it has been in operation only a few minutes. If a glass and metal film holder is placed in front of the boxlight close enough to transilluminate the image so that it can be photographed or projected, the film holder will experience a sharp temperature rise. For example, a black anodized aluminum frame and the glass it surrounds placed 6 inches in front of a 6000 watt box light have been found to undergo a 10.degree. F. temperature increase in 120 seconds from a "standing start" at room temperature. Such an increase is sufficient to change the dimensions of a film negative during its exposure with resultant blurring of the projected image and making the size of sequentially related images difficult to maintain. This potential size change becomes critical in a step-and-repeat operation, most often used in conjunction with a four color process where the congruent images must be of the same size if the half tones are to "rosette" properly over the expanse of a large press sheet.
Pulsed xenon light is an intense white light with a color temperature of about 6000 angstroms rich in ultra violet and infrared. It is unsuitable for any except very limited viewing with the human eye.